Verdict.ProbabilitySourceA number in [0,1] that is one value of a probability distribution.
The representation is shared with Confidence.t, but the two are not interchangeable: a probability is one member of a distribution, while a confidence is a claim about an answer as a whole. They never belong in the same expression, so they are separate abstract types.
Values are obtained only through of_float, which rejects NaN, infinities, and anything outside [0,1] with a message. When a server value fails that check while decoding, the rejection surfaces as a path-carrying Error.Decode naming the offending field, so an out-of-range value is never silently clamped.